1. Field of the Invention
The present invention relates generally to a field of bit-mapped image character recognition systems for machine readable forms and, more particularly, to a method of image pre-process assisting character and text recognition from a bit-mapped binary image or other binary or raster images inputted from a scanning device or the like, especially in the presence of distortion.
2. Prior Art
Segmentation and parsing methods are known in the art. Typically, such methods divide an image into homogeneous regions containing text or non-text objects and use a plurality of special computing procedures, each depending on a plurality of parameters, to analyze an object, detect and compensate distortion, if any appeared from external devices processing of the document.
According to the prior art the directions of text lines are used as guiding lines for horizontal direction estimation, supposing them to be horizontal.
A method and apparatus for detecting the skew angle of a document image is proposed in U.S. Pat. No. 5,563,403 Oct. 8, 1996 Bessho, et al. Skew angle determination is performed by the steps of determining a set of sampling points from an input document image and processing X and Y coordinates of the sampling points in order to calculate a regression coefficient of the sampling points. The skew angle of the document is determined using the regression coefficient. To evaluate a calculated skew angle which corresponds to the regression coefficient, a correlation coefficient is calculated and evaluated. Since coordinates of sampling points are obtained for a plurality of sets of data corresponding to different guiding lines or lines of characters, a histogram may be used to determine the most probable one.
The method can be used only with large text blocks and small values of skew.
A method and apparatus for document skew and size/shape detection is known from U.S. Pat. No. 5,818,976 Pasco, et al. Oct. 6, 1998 and U.S. Pat. No. 6,064,778 May 16, 2000 Pasco, et al. The invention relates in general to optical scanning and image processing, and relates more particularly to a document imaging system, which detects skew and/or detects size/shape of a document image. Preferred embodiments utilize a background with optical characteristics, which contrast with those of the scanned document. In one embodiment, a document imaging system generates scanning signals in response to optical characteristics of a medium such as a sheet of paper, detects transitions in the scanning signal which define points along one or more edges of the medium, establishes a skew angle between the detected edges and a reference orientation, and compensates for skew by modifying the scanning signals as required such that the resultant skew angle is substantially equal to zero. In another embodiment, a document imaging system detects one or more edges of a document, defines a polygon having sides substantially congruent with the detected edges, and establishes the size of the document in response to the polygon.
The invention can't be applied to the machine-readable forms, especially in the case of large skew and size/shape distortion.
A method for determining whether a scanned image is an original image or fax image is described in U.S. Pat. No. 5,828,771 Oct. 27, 1998, Bloomberg. An efficient image processing technique automatically analyzes an image scanned at 300 or greater dpi and measures an image characteristic of the input image from which it is possible to determine whether the image has ever been previously scanned or printed at low resolution at some time in its history. The technique is effective in classifying an image that was at one time embodied in paper form and scanned at a vertical resolution of 100 dpi or less, such as a facsimile document scanned in standard mode, or at 200 pixels/inch (referred to as “fine fax mode”.) The technique performs measurements on the pixels included in the vertical or horizontal edges of symbols contained in the input image, and produces a distribution of the measurements. A numerical interpretation of the measurement distribution data is used to classify the image.
The invention may be applied to only a small percentage (e.g., 7%) of a document image as long as the subimage selected contains symbols such as characters.
An image reading apparatus having a function for rectifying an image distortion caused by a curvature of a document surface is described in U.S. Pat. No. 6,014,470 Jan. 11, 2000, Matsuda, wherein the image distortion is rectified by duplicating image data that is a line of pixels in accordance with a varied amount of heights of a document surface, and a data conversion that is a image processing procedure in which the number of bits for pixels is reduced, is executed for the rectified image data. The reducing a memory capacity needed for the distortion rectification can be achieved without lowering picture quality.
The method has a limited application since it could rectify only image distortions caused by curvature of the document surface.
A specialized address reading method and apparatus for recognizing a receiver address on a surface of mail is proposed in U.S. Pat. No. 6,470,091 Oct. 22, 2002 Koga, et al. An image of the surface of the mail is input and segmented into at least one character string candidate. At least one address area candidate is extracted from the image based upon the segmented character string candidate. One address area candidate extracted from the image is selected as a receiver address by analyzing each address area candidate based on predetermined position information indicating a usual position of a receiver address area, character direction information indicating a character direction of a character string appropriate for the predetermined position information, and key character string information indicating a character string most likely existing in a receiver address. Characters in character strings of the selected address area candidate are recognized as a receiver address.
The method supposes little or none distortion or skew of scanned image and bases on the single type model document—postage envelope.
All known methods could not be used to compensate distortion of a considerable value (more then 10% of the initial image size).